Method And Device For Continuously Forming Optical Fiber Connector Glass And Other Close Tolerance Components

ABSTRACT

A method and device for making high precision glass forms ( 110 ). A glass rod ( 1 ) is pushed into a melting tube ( 47 ) and the glass form is pulled from the chamber. Preferably, both the push rate and the pull rate are controlled. Fiber optic glass ferrules and other components manufactured by the use of this invention have precision dimensions that fall well within the tight dimensional tolerances required for ferrules and others.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional application Ser.No. 60/550,464, filed Mar. 4, 2004, hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to the production of high precision glassarticles for optical fiber connectors and for other uses, and to amethod of manufacture and a machine for carrying out the method.

BACKGROUND ART

Previous methods for making high precision glass tubing employ thewell-known redraw technique in which a close tolerance redraw blank tubeis drawn down to a smaller size on a mandrel to make such articles asglass ferrules. See for instance U.S. Pat. Nos. 4,850,670, 5,295,213,5,314,517, 6,098,428, and 6,810,691.

In all of the various redraw processes, the dimensional characteristicsof the tubular starting blanks substantially control all of the finaldimensions of the redrawn tubing. Such things as roundness,concentricity of inner bore to outer diameter and the ratio of innerbore size to the outer diameter can not be changed during redraw, and asa consequence, the greatest proportion of the cost to make redraw tubinglies in the original blank preparation costs and the very inefficientbatch type non-continuous redraw operation.

My previous method of making high precision glass tubing, described inU.S. Pat. No. 3,401,028, employs bulky and expensive equipment andgenerally is incapable of forming glass tubing having the high precisionrequired for many modern applications, such as the manufacture of glassferrules or connectors for optical fibers. These applications mayrequire precise inside and outside dimensions, wall thickness,roundness, and concentricity, all measured in nanometers, for exampleone hundred nanometers or less, sometimes ten nanometers or less.

Other methods of forming glass tubing are shown in U.S. Pat. Nos.4,350,513 and 4,372,771.

The patents mentioned above are hereby incorporated by reference.

SUMMARY OF THE INVENTION

The present invention provides a method whereby a high precision redrawblank is not used, but rather commercial grade round glass rod is usedas the starting material. The rod may be continuously drawn or in cutlengths. This invention provides a way to feed, extrude and pull moltenglass tubing and rod under pressure from a die. Errors in all thecritical dimensions of the resulting product may, if desired, becontinuously corrected by an automatic feedback system. As a result ofthis ability to change dimensions on the fly, it is no longer necessaryto build into the starting material extremely costly high precisiondimensional characteristics.

The elimination of a high precision starting blank and the ability torun continuously eliminates as much as 90% or more of the cost of makingredraw tubing and gives a large commensurate improvement in the highprecision size tolerances.

The articles made by continuously drawing glass tubing according to thisinvention, are controlled for outside diameter, inside diameter,roundness, wall thickness and axial center of inside diameter inrelation to the outside diameter by both automatic and manually adjustedparameters.

Both hollow and solid glass articles can be manufactured by utilizingthe teachings of the instant invention. Single bore and double bore, aswell as multi-bore tubing for such applications as fiber optic connectorferrules and sleeves and photonic band gap materials can be made withtolerances measured in nanometers, typically less than 100 nanometers,sometimes on the order of ten nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view in side elevation of one illustrativeembodiment of apparatus according to the invention for carrying out oneillustrative embodiment of the methods of the invention.

FIG. 1A is a view in side elevation of a rod holding system portion ofthe apparatus of FIG. 1.

FIG. 1B is a view in side elevation of a rod feeder portion of theapparatus of FIG. 1.

FIG. 1C is a view in side elevation of a melting chamber and formingsystem portion of the apparatus of FIG. 1.

FIG. 1D is a view in side elevation of a pulling system portion of theapparatus of FIG. 1.

FIG. 2 is a cross-section taken along line 2-2 of FIG. 1A.

FIG. 3 is a top view of the rod feeder shown in FIG. 1B.

FIG. 4 is a cross-section taken along line 4-4 of FIG. 3.

FIG. 4A is a cross-section corresponding to FIG. 4, showing an upperdrive roller in a raised position.

FIG. 5 is a somewhat schematic view of the apparatus of FIG. 1, showingcontrol systems and water cooling systems for the illustrativeapparatus.

FIG. 6 is a view in side elevation of the melter portion and a die ofthe forming system of FIG. 1C with insulation removed and without glassin the system.

FIG. 7 is a cross-sectional view of the melter and forming portion ofFIG. 6.

FIG. 8 is a view in perspective of the melter chamber of FIG. 6.

FIG. 9A is a view in side elevation of an alternative embodiment of acylindrical inner forming tube, a view in end elevation of a die for usetherewith, and a view in end elevation, not to scale, of a drawn shapeformed therewith.

FIG. 9B is a view in side elevation of an alternative embodiment of amultiple-hole inner forming tube, a view in end elevation of a die foruse therewith, and a view in end elevation, not to scale, of a drawnshape formed therewith.

FIG. 9C is a view in side elevation of an alternative embodiment of arectangular inner forming tube, a view in end elevation of a die for usetherewith, and a view in end elevation, not to scale, of a drawn shapeformed therewith.

FIG. 10 is a fragmentary view in end elevation of the melter portion ofFIG. 6, taken as indicated by the line 10-10 of FIG. 6.

FIG. 11 is a fragmentary sectional view taken along line 11-11 of FIG.1C of a laser micrometer for use in the illustrative apparatus.

FIG. 12 is a fragmentary sectional view taken along line 12-12 of FIG.1C of a microscope measuring system for use in the illustrativeapparatus.

FIG. 13 is a fragmentary sectional view taken along line 13-13 of FIG.1D of another laser micrometer for use in the illustrative apparatus.

BEST MODES FOR CARRYING OUT THE INVENTION

The following detailed description illustrates the invention by way ofexample and not by way of limitation. This description will clearlyenable one skilled in the art to make and use the invention, anddescribes several embodiments, adaptations, variations, alternatives anduses of the invention, including what I presently believe is the bestmode of carrying out the invention. As various changes could be made inthe above constructions without departing from the scope of theinvention, it is intended that all matter contained in this descriptionor shown in the accompanying drawings shall be interpreted asillustrative and not in a limiting sense.

Referring now to the drawings, FIG. 1 shows one illustrative embodimentof an apparatus A in accordance with the present invention, for use incarrying out illustrative embodiments of methods of the presentinvention. The device is shown in more detail in FIGS. 1A-1D, eachshowing a portion of the device of FIG. 1.

As described in more detail hereinafter, FIGS. 1A and 2 show a rodholding portion of the illustrative apparatus A, into which commercialquality glass rods 1 are loaded manually for feeding with minimal forceinto a rod feeding portion of the apparatus A, shown in FIGS. 1B, 3, 4,and 4A. The rod feeder portion feeds rods 1, butted end-to-end, withcontrolled speed or force, and without breaking the ends of butted rods1, into a melting chamber and forming system portion of the apparatus.The melting chamber and forming system portion is shown in FIGS. 1C, 6,7, 8, and 10. This portion includes a melter 27 which includes an inletfunnel section 40, a restriction area 46 which forms a semi-molten glassseal with the rod 1 as it melts, a melting tube 47 which includes andforms an extension of the restriction area 46, a transition block 49which supports an inner forming tube 51 in an offset outlet tube 53, anoutlet flange 33, and electrodes 43 and 52. A temperature sensor 32, inthe form of a pyrometer, is directed at the melting tube 47 through asmall opening in an insulative jacket 63 around the melter 27.Thermocouple 41 monitors inlet temperature, thermocouple 48 monitorsmelting tube temperature, and thermocouples 58 and 59 monitor outlettemperature. A die 68 is adjustably mounted to the outlet flange 33 bysupport structure 111-117. A draw down 73 of semi-molten glass emergingfrom the die 68 is measured by parts of the forming system shown inFIGS. 11 and 12 as it is pulled into a shape 110, illustratively smalldiameter tubing, by a pulling system portion of the apparatus as shownin FIGS. 1D and 13. The pulling system portion also cuts the tubing intosections. The operation of each portion of the illustrative apparatus iscontrolled by a control system shown in FIG. 5. The control system may,if desired, control the operation of the rod feeding system, the speedat which the rod 1 is fed into the melting chamber, the speed at whichthe tubing 110 is pulled from the forming system, the temperature ofzones within the melting chamber, and the rate of cooling of the drawdown 73 and tubing 110. Adjustment of these parameters allowsunprecedented control of the finished shape 110, as do physicaladjustments of the die 68 and the inner forming tube 51.

In FIG. 1A, reference numeral 1 designates a solid, round glass rod cutto lengths with square ends being fed to the rod feed system. Rollerguides 2, 3, and 4 support the incoming glass rod and hold it on thecenter of the rod holding system as the rod is being pushed forward bymotor driven roller 6. The rod end activates photocells 7, 8, 9 and 10as it passes by each cell to start timers 11, 12, 13 and 14,respectively. These timers signal air cylinders 23, 24, 25, and 26 inthe rod feed system of FIGS. 1B, 3 and 4 to lift momentarily andsequentially to allow the rod ends to pass through pinch rollers 15, 16,17 and 18 and drive rollers 19, 20, 21 and 22, so that rod ends will notcrack during their passage through them. FIG. 4A shows the first pinchroller 15 raised to allow a rod end to pass through it, while pinchroller 16 continues to drive the lead rod 1. Because only one pinchroller is raised at a time, the remaining rollers will continue to driveboth the rods 1 toward the inlet funnel 40. Air cylinders 23 24, 25 and26 that are controlled by electric solenoids and air pressure regulatorsactivate the pinch rollers. Pinch and drive rollers are driven by motor30 through right angle gear boxes 31. End of rod detector 5 is aphotocell that gives an alarm when the end of the glass rod 1 isreached, in order for the next rod to be loaded into the rod holdingsystem. The output voltage of controller 38, which is connected totemperature sensor 32, controls the speed of these rollers.

As the rod 1 is fed into the inlet funnel 40 it begins to soften. As itenters the restriction area 46, the outer portion of the rod forms asemi-molten ring which forms a seal with the melting tube 47. Becauseincoming glass in contact with the melting tube 47 cools it,instantaneous changes in the temperature of the tube are indicative ofinstantaneous changes in the mass of the glass rod being fed into themelting chamber. It has been found that by placing a temperature sensor32 on the outside of the melting tube 47 near restriction area 46, asignal through controller 38 can adjust the speed of motor 30. Thiscontrols the feed rate of the rod, and consequently, assists in theprecise control of the final product dimensions. The precise position ofthe temperature sensor 32 may be varied somewhat around the restrictionarea 46.

Glass rod 1 is guided into the upstream end of inlet funnel tube 40 bycenter guide sleeve 44, which is held by bracket 39. Inlet funnel tube40 is also held in place by bracket 39, which is locked in place aftermelting tube 47 is at running temperature and will not undergo any morethermal expansion.

Inlet funnel tube 40 is heated by muffle furnace 42 and its temperatureis controlled by thermocouple 41 in combination with controller 36 andSCR 35. This preheats the glass rod 1 before it enters the restrictionarea 46.

There are four strap electrodes 43 set 90° apart and four strapelectrodes 52 set 90° apart that are connected to main power electrodes60 and 61 coming from transformer 64. The main power electrodes areinsulated from each other except through the body of the melter 27. Thecontact ends of strap electrodes 43 and 52 are clamped by plates 60′ and61′ to main power electrodes 60 and 61, respectively, and are cooled bywater flowing through cooling rings 103 and 102, respectively. Thecurrent between the electrodes 43 and 52 through the body of the melter27 raises the temperature of the melting tube 47, transition block 49,and the upstream portion of the outlet section 53 by resistance heating.Strap electrodes 43 at the inlet end are made narrower than strapelectrodes 52 to establish a thermal gradient between the ends of themelting tube 47. Thermocouple 48, controller 65, and SCR 66 controltransformer 64. Constant voltage power supply 67 prevents sudden linevoltage changes from affecting melting tube 47 temperatures.

The inlet of the melting tube 47 is tapered at restriction area 46 at aconical angle of about 1° (included angle of about 2°), and theremainder of the melting tube 47 is of uniform inside diameter. Both thedownstream end of the restriction area 46 and the melting tube 47 aresmaller in diameter than the smallest rod that will be fed into thesystem.

The melting tube 47 is resistance heated by transformer 64 between mainpower electrodes 60 and 61. The wall of melting tube 47 is approximately30% thicker than restriction area 46 and the downstream end of inletfunnel tube 40 to cause these areas to run hotter than melting tube 47.This makes up for heat loss by the strap electrodes 43, as glass rod 1is pushed through inlet funnel tube 40 and through restriction area 46.

Insulation 63 is used on the melting tube and throughout the melting andforming system to thermally insulate warn areas. Ball bushing 45 allowsmelting tube 47 to expand and contract with temperature changes.Flexible electrical cable 62 allows main electrode 60 to be split sothat it can move with expansion and contraction as well.

The inlet funnel tube 40 preheats the glass rod 1 sufficiently to softenthe exterior portions of the glass rod 1 upstream of the restrictionarea 46. The restriction area 46 helps maintain the glass rod 1 oncenter. The restriction area 46, which has a reduced section about 0.5%to about 5% smaller in diameter than the smallest round glass rod 1being fed to the inlet funnel tube 40, melts the rod and forms acontinuous seal between the rod and the wall of the melter 27. A smallring of molten glass forms on the upstream side of the restriction area46. That ring is continuously drawn through the restriction area 46 bythe relatively cool glass rod 1, thereby reducing entrainment of air bythe rod. The continuous seal formed between the glass rod 1 and therestriction area 46 also prevents flow of glass from the melting chamberback into the inlet funnel tube 40 and allows for relatively highpressures to be built up in melting tube 47 and outlet section 53. Thepushing force of the incoming rod 1, typically about fifty pounds(twenty-three kilograms), causes pressure in the melting tube 47 and theoutlet section 53. This pressure prevents expansion of any trapped airin the molten glass and reduces or prevents the formation of air bubblesor airlines in the finished product. It has also been found that placingthe inlet section of the melting tube 47 below the outlet section 53helps to minimize air bubbles in the finished tube.

By cooling outlet section 53 and die 68, the pressure in the meltingsystem allows the viscosity of the extruded glass to be high enough sothat gravitational forces acting on the horizontal glass do not causeany significant sag or deformation of the finished product.

A hollow inner forming tube 51 is positioned in the outlet section 53 ofthe melting system. The inner forming tube 51 is formed of a drawnhollow tube having a shape machined in a screw-on tip at its downstreamend. The downstream section of the inner forming tube 51 is positionedwithin the die 68 and forms the bore of the draw down 73 and thus of thefinal glass tube 110. As used herein, the term “draw down” describes thesemi-molten shape of glass emerging from a melter portion of theinvention. The draw down in the preferred embodiment is a hollow shapeemerging from the die 68 at the outlet of the section 53 and having aratio of inner dimension to outer dimension generally the same as thefinished glass tube produced by the preferred process and apparatus ofthe invention.

The downstream end of the inner forming tube 51 is preferably positionednear the outlet orifice of the die 68. The inner forming tube 51communicates with the atmosphere through its upstream open end. Theupstream end of the inner forming tube 51 can also be connected to apressure/vacuum source 74 to affect the shape or dimensions of the finalproduct. If desired, other fluids than air can be connected to theupstream end of the inner forming tube 51 to affect the characteristicsof the final product 110. The upstream end of inner forming tube 51 isconnected to actuator 54 through a gland 50 in the transition block 49.

Gland 50 is kept at a semi-molten temperature that will not leak glass,but still allows motion of the inner forming tube 51. Inner forming tube51 is supported near its open upstream end by actuator 54 and issupported inside the outlet section 53 by inner forming tube spokes 55.Actuator 54 can move inner forming tube 51 radially through semi-moltengland 50 to position the inner forming tube 51 on the center of die 68and also longitudinally as shown by dial indicator 79 to controlrelative sizes of the inner diameter (ID) and outer diameter (OD) of theglass extruding from die 68. Inner forming tube 51 can also be rotatedfor alignment of non-circular cross-sections. The gland 50 is typicallyformed by an opening in the wall of the transition block 49 about 0.02to 0.1 mm larger than the outer diameter of the inner forming tube 51.Because of the thickness of the wall in which the gland 50 is formed,the temperature of the gland is somewhat cooler than the average walltemperature of the melter 27.

The temperature of outlet section 53 is controlled by split clamshellfurnaces 56 and 57, thermocouples 58 and 59, controllers 69 and 71 andSCR 70 and 72. The two furnaces are split so that different temperaturescan be set above and below outlet section 53 to adjust the shape in thedraw down 73.

Die 68 can be adjusted by using yoke 111 and yoke pins 112 to push thedie in the horizontal and vertical directions. Once die 68 is adjusted,it is clamped in place with holder 113, ceramic spacer 114, end ring 115and nuts 116. These nuts are threaded on shafts 117 that protrude fromthe outlet flange 33.

The laser micrometer 37 measures draw down 73 and is held by bridge 76on opposite sides of glass covered draft box 77. Bridge 76 can be movedby screw assembly 78 to position laser micrometer 37 as close aspossible to die 68 so that there is minimal delay in seeing changes inthe draw down 73 size. Microscopes 81, set at a 90° angle to each other,are used to view the inner capillary bore on microscope display 82 forcentering of the issuing glass tube 110, and to adjust the relative sizeof the inner capillary bore by manipulating actuator 54 and moving innerforming tube 51 while the machine is in operation and gland 50 is hot.

Draft box 77 shields the issuing glass from air disturbances, whileallowing viewing by laser micrometer 37 and microscopes 81. Split hingedwater cooled box 83 prevents drafts from disturbing glass dimensionsbefore it has set to final size as it is being pulled by upper pullroller 87 and lower pull roller 88. Laser micrometer 84 is used to sensethe final outer diameter of glass tube 110 as the glass is being pulledthrough vertical guide rollers 105 and horizontal guide rollers 106.Laser micrometer 84 is held by bridge 86 on opposite sides of glass tube110. Output from laser micrometer 84 and/or laser micrometer 37 can beused to sort glass tube 110 appropriately in a glass cutting/sortingsystem 89. Cutting and sorting systems are well known in the art. Ascore-and-crack cutting system has been found to be operative.

The pull roller system (FIG. 1D) is operated so that pull roll motor 85drives upper pull roller 87 and lower pull roller 88 at the same speedwhile they hold the glass tube 110 being pulled. Laser micrometer 37monitors the size of draw down 73 and sends a signal to controller 99 tovary the speed of the pull rollers to keep a final fixed dimension ofthe glass tube 110.

Constant temperature water cooling comes from tank 100 and pump 101through split hinged water cooled box 83 to electrode cooling ring 102to electrode cooling ring 103 and back to chiller 104 and to tank 100for constant temperature conditioning and back to pump 101.

The five process instruments are Honeywell UDC-3300 Digital Controllers,Fort Washington, Pa. The motors and actuators are Aerotech, Pittsburgh,Pa. The laser micrometers are Keyence LS-5000 series made by KeyenceCorporation of America, Woodcliff Lake, N.J. The temperature sensor is apyrometer from Engelhard, Fremont Calif. There are several manufacturerswho make this type of equipment so those mentioned are not unique.

Illustrative methods of continuously forming high precision glass tubewith the illustrative apparatus A was carried out as follows.

For process development purposes, all metal parts that were in contactwith molten glass were made of 310 stainless steel made by RolledAlloys, Temperance, Mich.

The glass rod feed stock was 11 mm SG 10 glass, made by Sylvania,Versailles, Ky. Outer diameter tolerance was ±0.15 mm. The small end ofthe inlet tunnel tube 40 on the melter was 11.5 mm in diameter, and therestriction area 46 was 10.79 mm in diameter. The outlet die 68 diameterwas 12.50 mm, and the inner forming tube 51 was 1.0 mm in outer diameterat its downstream end, 6.3 mm in outer diameter through most of itslength, and 0.63 mm in inner diameter through its entire length.

After heating the machine to start feeding glass rod through the inletfunnel tube 40 with input furnace at approximately 900° C., the rod wasfed at a sufficient rate to make 1.25 mm O.D. tubing with a 125 micronI.D. bore at a pull rate of eight meters per minute. The glass meltingtube 47 was approximately 1050° C., the split clamshell furnaces 56 and57 were approximately 1020° C. and the finished tubing had sizetolerances within those required for fiber optic glass ferrules. Thecurrent between the main power electrodes 60 and 61 is about 1500 ampsat 1.5 volts.

The adjustments to the position of the inner forming tube 51, taken withthe rate at which the glass rod 1 is pushed, the rate at which the finalglass tube 110 is pulled, and the temperature of the melting chamberoutlet section 53, give unprecedented control of the dimensions,roundness and concentricity of the final glass tube 110.

The method and apparatus of the present invention have numerousadvantages over redraw techniques. They can reduce the cost ofmanufacture by 90% or more. The use of a high pushing force with theglass rod 1 (above about five kilograms, preferably about fifteen toforty kilograms) allows glass to be made continuously without airlines,and gives much closer dimensional control and sharper shapes thanredraw. Square corners and flat surfaces are easily formed. Double andmultiple bore shapes can be continuously formed in this one-step processas opposed to the many steps and complications of other inventions suchas the process suggested by aforementioned U.S. Pat. No. 6,810,691. Themethod and apparatus provide the ability to run continuously from asource of glass rod such as made by the well-known Vello process andeliminates the problems caused by welding rods or performs together. Itwill be understood, however, that the relatively inaccurate rods formedby the Vello process may, if desired, be welded together to form acontinuous glass feed, or the rods may, if desired, be reformed tocloser dimensions by melting and extruding them from a die. Because thepresent invention can position both the incoming glass rods and thefinished tube horizontally, it can eliminate the need for drawingtowers. This invention can also be positioned vertically or at any anglein a tower for glass production if desired.

As numerous variations will be apparent to those skilled in the art, theforegoing disclosure is to be understood as exemplary and not aslimiting the scope of the invention, whose scope is to be determinedsolely by the following claims.

Merely by way of example of changes that could be made, other types ofglass may be utilized. To produce tubing from 7740 Borosilicate glass, aplatinum alloy such as PT-20RH is preferred for all the metal parts thatcome into contact with molten glass, and the temperatures on the variousparts of the apparatus as stated above would all be increased bysomewhat more than 200° C. In the same manner, for quartz products andother high temperature applications, approaching 2000° C., iridium orother refractory metals such as molybdenum or tungsten can be used asthe material for the metal parts. A glove box or nitrogen curtains canbe used to contain the atmosphere around the melting chamber portion ofthe system if needed.

As shown in FIGS. 9A, 9B, and 9C, the shape formed by the illustrativeapparatus can easily be changed by changing the die 68 and inner formingtube 51, and the inner forming tube may be formed as a single piece. Thesize of the rod 1 and the size of the shape 110 can be varied widely.

The ratio of the outside diameter to inside diameter of the tubingproduced by the example was primarily for illustrative purposes and notto limit in any way what this ratio might be on any particularproduction run. Other shapes, such as rectangular tubing may be producedwith a rectangular die 68 and rectangular inner forming tube 51 as shownin FIG. 9C. Eliminating or moving rearward the inner forming tube 51 mayform solid glass rods of any cross section formed to close tolerances. Acluster of inner forming tubes 51 may also be utilized to produceproduct with multiple lumens as shown in FIG. 9B. Such a cluster couldbe of different diameters as well as the same diameter. This approachmakes possible the production of a wide variety of photonic band gap orphotonic crystal fibers and multiple-fiber connectors.

Tubes or rods of continuously varying diameters, with any desired taper,may be formed by periodically changing the feed rate and/or the pullrate. The relative size of the inner bore of the tube 110 can be variedby systematically periodically moving the inner forming tube 51 forwardand back.

Rubber cog belts can replace upper pull roller 87 and lower pull roller88. By setting the rubber cog belts at an angle to each other in thehorizontal plane, the round glass can be caused to rotate as it ispulled from the die, if so desired.

Rubber cog belts or other methods familiar to those skilled in the artcould replace the pinch and drive rollers in the rod feed system.

The use of the laser measurement device that responds to glass outsidediameter after the glass has solidified may not in all cases be needed.When utilized, a measurement of the finished glass tube may be made atany point after the tube leaves the outlet orifice of the melter.Preferably, the measurement of the inlet temperature is utilized tocontrol the push rate of the feed rod, and measurement of the finishedtube is utilized to control the pull rate on the tube. It will beunderstood, however, that either of these measurements can be utilizedfor either purpose or both, and algorithms will immediately occur tothose skilled in the art for mixing the measurement signals to controlboth push rate and pull rate. Because the system has very littlehysteresis, the amount of control required is greatly reduced. It willfurther be understood that although it is highly desirable to maintainthe temperature of the melting chamber constant, it is possible toadjust both push and pull rates to compensate for variations intemperature. For looser tolerances on the final product, rod feedspeeds, melting chamber temperatures and pull roller speeds can be setmanually with no feedback control. If higher precision is required, theinput glass rod 1 can be ground to close OD tolerances.

The glass size measuring and control system may use other than lasers tocontrol the final glass size. The system may use various temperaturesensors such as thermocouples and pyrometers interchangeably to senseand control the temperatures throughout the device.

The glass melting and forming portions of the apparatus can be heated bynot only by resistance heating, but also by radiant, induction heatingor by other methods known in the art to produce a desired temperatureprofile or gradient.

The glass cutting/sorting portion 89 of the pulling system can useseveral methods known in the art, including by way of illustration flamecutting, laser cutting, and diamond sawing.

Die 68 may be formed in various shapes other than such as the preferredplate, such as a cone or a tube to form the outside dimensions of theproduct.

The center guide sleeve 44 may be eliminated or replaced by guiderollers. The shape of the inlet funnel tube 40 may be varied toaccommodate the shape of the feedstock and the amount of pre-heatingrequired.

Motors throughout the system may be servo, stepper or various typescapable of providing consistent rotation speed and control.

The diameter of the rod at the restriction area 46 may be determined inother ways. For example, a strain gauge mounted to the restriction orits mountings could be utilized to measure the pushing force of theglass rod 1. The torque required to drive the glass rod 1 may also beutilized to determine the size of the rod at the restriction. Thediameter of the glass rod 1 may be directly determined at a point beforethe restriction, as by an array of laser interferometers, and the rateof feed varied in accordance with the predicted time of arrival (using ashift register) of variations in diameter at the restriction. Theillustrative method of measuring temperature at the restriction is amore direct way of determining mass at the inlet of the chamber,however, and is therefore presently preferred.

Although the illustrative machine preferably utilizes glass, it will beunderstood that in principle the machine and method may be used forforming any heat-softenable material.

These variations are merely illustrative.

1-10. (canceled)
 11. A method of making a glass shape comprising a stepof providing a heating chamber, the heating chamber having a singleinlet and a single outlet, a step of pushing a solid glass rod into theinlet and a step of pulling a shape from the outlet, wherein the inletcomprises a heated cone, the cone melting the exterior of the rod andforming a molten glass seal at the inlet.
 12. A method of making a glassshape comprising a step of providing a heating chamber, the heatingchamber having a single inlet and a single outlet, and a step of pushinga solid glass rod into the inlet, wherein the inlet comprises a heatedcone, the cone melting the exterior of the rod and forming a moltenglass seal at the inlet.
 13. The method of claim 12 wherein the inlethas a diameter slightly smaller than the diameter of the rod.
 14. Themethod of claim 13 wherein the rod has a diameter, which varies at least0.5% and no more than 5%.
 15. The method of claim 14 wherein the inlethas a diameter 0.5% to 5% smaller than the smallest diameter of the rod.16-26. (canceled)
 27. An apparatus adapted to form a hollow tube, theapparatus comprising a heated chamber having an outlet, a die in theoutlet, and a hollow inner forming tube extending from the vicinity ofthe outlet, within an inside dimension of the die, through a gland in awall of the chamber.
 28. The apparatus of claim 27 further comprising anadjustment device operatively attached to a part of the hollow innerforming tube outside the chamber.
 29. The apparatus of claim 28 whereinthe inner forming tube is straight, the apparatus further comprising aninlet passage having an axis parallel to the inner forming tube andoffset from the inner forming tube.
 30. The apparatus of claim 27wherein the chamber is filled with molten glass, the glass being cooleradjacent the gland and adjacent the die than the average temperature ofthe glass in the chamber, the gland forming a seal of glass between theinner forming tube and an opening in a wall of the chamber.
 31. Anapparatus for feeding glass rod sections comprising a plurality of feeddrives, at least one of the feed drives being biased into engagementwith the rod, a sensor for detecting rod section ends, and a mechanismfor varying the bias of the at least one feed drive in response to thesensor to protect the rod ends.
 32. The apparatus of claim 31 whereinthe rod ends are abutting. 33-36. (canceled)
 37. A method of controllingthe rate at which a solid rod of heat-softenable material is fed througha heated restriction, the restriction softening at least an outerportion of the rod, the method comprising a step of determining changesin temperature at the restriction, and a step of controlling the rate offeeding the rod in response to changes in temperature at therestriction, wherein the restriction is the inlet of a melting chamber.38. The method of claim 34 wherein the melting chamber includes anoutlet, the material forming a draw down at the outlet. 39-63.(canceled)